PROJECT SUMMARY HIV infects macrophages in the spinal cord and brain, often leading to clinical symptoms even with antiretroviral treatment (ART). In spinal cord and brain, macrophages (M?s) may serve as long-term cellular reservoirs of latent HIV. Replication competent HIV can emerge from these latent reservoirs if ART is stopped. SIV replicates in both spinal cord and brain at equivalent levels during acute infection; however, the specific cellular targets of SIV in the CNS have not been classified throughout infection, including during latency and rebound after interrupting ART. In contrast with acute infection, viral dynamics are significantly different in these distinct CNS compartments after stopping ART and tracking SIV rebound. SIV RNA is readily detectable in spinal cord in the first weeks after stopping ART; in contrast, most SIV-infected animals do not have detectable SIV RNA in the brain. The disparity in rebound replication between spinal cord and brain may result from unique cellular immune responses by M? and astrocytes in these distinct CNS compartments. Most SIV- infected macaques receiving ART have no detectable SIV RNA in spinal cord or brain, CSF, or plasma. Proviral DNA and immune responses persist nonetheless in both spinal cord and brain. Intriguingly, the nature of the sustained immune response differs: in the spinal cord, elevated expression of both GFAP and CCL2 point to sustained astrocyte activation; in the brain, elevated CD68 and TNF? levels indicate persistent brain M? activation. The central premise of this proposal is that spinal cord M?s serve as a distinct SIV reservoir in the CNS that reactivates rapidly after stopping ART. Cellular neuroimmune responses in the spinal cord differ from the brain during latency and reactivation after stopping ART. In particular, coordinate regulation of spinal cord M? and astrocyte immune responses intrinsically differ from brain responses. Specific Aim 1 is to determine whether spinal cord M?s constitute a unique SIV reservoir in the CNS by identifying SIV-infected CNS cell populations during 1) acute SIV infection, 2) prolonged latency with ART, and 3) viral rebound after stopping ART. Specific Aim 2 will identify the neuroinflammatory responses in the spinal cord and brain during 1) acute SIV infection, 2) prolonged latency on ART, and 3) viral rebound after stopping ART. Gene expression profiling will be performed using nCounter immunoregulatory assays; single cell RNA-seq gene expression profiling performed on single cells isolated from spinal cord and brain including M?s and astrocytes will complement targeted gene profiling. Specific Aim 3 is to determine whether depleting spinal cord and brain M?s during latency and continuing after stopping ART by treatment with the CSF1R inhibitor PLX3397 alters CNS SIV replication during SIV rebound from latency. M? depletion will complement SA1 studies on CNS M?s as latent reservoirs and SA2 studies on spinal cord gene expression profiles in latency and during SIV rebound to advance our understanding of the role that spinal cord M?s play in HIV latency.